The present invention relates to a catalyst and process for methanol and/or dimethyl ether to hydrocarbons, particularly light olefin rich in ethylene.
There is a growing need for light olefin, especially ethylene and propylene, for a variety of uses making it desirable to develop sources of the olefin additional to the conventional source, crude oil. One such additional source is methanol and/or dimethyl ether which can be catalytically converted over certain zeolite catalysts to olefin-containing hydrocarbon mixtures. For example, U.S. Pat. No. 3,911,041 discloses that methanol can be converted to C.sub.2 -C.sub.4 olefin by contacting the methanol at a temperature of 300-700.degree. C. with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12, such as ZSM-5, and which contains at least about 0.78% by weight of phosphorus incorporated in the crystal structure of the zeolite. U.S. Pat. No. 4,088,706 discloses that methanol can be converted to a hydrocarbon mixture rich in C.sub.2 -C.sub.3 olefin and mononuclear aromatics, particularly p-xylene, by contacting the methanol with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12 and which has been modified by the addition of an oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus. U.S. Pat. No. 4,849,573 teaches that the use of zeolites with a Constraint Index of 1-12 and a silica to alumina molar ratio of 298 to 2000 increases the light olefin yield in the conversion of methanol to hydrocarbons.
U.S. Pat. Nos 4,049,573 and 4,088,706 disclose that modifying Constraint Index 1-12 zeolites, such as ZSM-5, with oxides of boron or magnesium, either alone or in combination with an oxide of phosphorus, increases the yield of p-xylene in the catalytic conversion of methanol to olefin and aromatics.
U.S. Pat. No. 4,480,145 discloses that the ethylene yield in the catalytic conversion of methanol over ZSM-5 can be increased by moderating the diffusivity of the zeolite by the use of the large crystal form of the zeolite and by silica "stuffing" of the zeolite pores. This patent also discloses that by steaming the zeolite at 180-820.degree. C. to reduce its alpha activity to 6-10 the cycle life and methanol conversion activity of the zeolite can be increased.
However, none of the prior art catalysts exhibit the combination of ethylene selectivity and hydrothermal stability desirable in a commercial methanol conversion process, particularly a fluid bed process with continuous oxygen regeneration. According to the invention, it has now been found that certain porous crystalline materials having specific and closely-controlled diffusion characteristics, such as can be obtained by severe steaming of phosphorus-containing ZSM-5, exhibit unexpected hydrothermal stability and enhanced selectivity to C.sub.2 to C.sub.4 olefin, and particularly to ethylene, when used to convert methanol and/or dimethyl ether.